1. Field of the Invention
The present invention relates to a developing device for use in an electrophotographic image forming apparatus and to an image forming apparatus incorporating the developing device. More particularly, the present invention relates to a trickle developing device that gradually supplies fresh developer and gradually discharge deteriorated developer and to an image forming apparatus incorporating the developing device.
2. Description of the Related Art
As developing systems employed for electrophotographic image forming apparatuses, the one-component developing system in which toner is used as the main component of the developer and the two-component developing system in which toner and carrier are used as the main components of the developer are known.
The two-component developing system that uses toner and carrier, in which the toner and carrier are charged by friction contact therebetween to predetermined polarities, has a characteristic that the stress on the toner is less than that in the one-component developing system that uses a one-component developer. Since the surface area of the carrier is larger than that of the toner, the carrier is less contaminated with the toner attached to the surface thereof. However, with the use for a long period, contamination (spent) attached to the surface of the carrier increases, whereby the capability of charging the toner is reduced gradually. As a result, problems of photographic fog and toner scattering occur. Although it is conceivable that the amount of the carrier stored in a two-component developing device is increased to extend the life of the developing device, this is undesirable because the developing device becomes larger in size.
To solve the problems encountered in the two-component developer, Patent document 1 discloses a trickle developing device being characterized in that fresh developer is gradually replenished into the developing device and developer deteriorated in charging capability is gradually discharged from the developing device, whereby the increase of the deteriorated carrier is suppressed. The developing device is configured to maintain the volume level of the developer inside the developing device approximately constant by discharging an excessive amount of deteriorated developer using the change in the volume of the developer. In the trickle developing device, the deteriorated carrier inside the developing device is gradually replaced with fresh carrier, and the charging performance of the carrier inside the developing device can be maintained approximately constant.    [Patent document 1] Japanese Patent Application Laid-Open Publication No. Sho 59-100471
However, since the volume, that is, the level, of the developer inside the developing device, changes depending on the state of the developer inside the developing device, that is, the concentration of the toner and the deteriorated state of the carrier, the ratio of the ingredients constituting the developer becomes different even if the volume of the developer remains the same.
The concentration of the toner inside the developing device is detected using, for example, a toner concentration detecting sensor that detects the permeability of the developer. For this reason, the measurement accuracy of the toner concentration detecting sensor is not sufficiently high, and the toner concentration indicated as a measured value may be different from the true toner concentration. In addition, the toner concentration obtained using the toner concentration detecting sensor may indicate a toner concentration different from the true toner concentration depending on the filling state of the developer around the toner concentration detecting sensor and the changes in the ambient environment of the image forming apparatus.
Because of various factors such as those described above, the toner concentration obtained using the toner concentration detecting sensor may be detected to be higher than a reference toner concentration that is assumed to be appropriate. Since the trickle developing device is controlled such that the volume level of the developer inside the developing device is maintained approximately constant, if the toner concentration is detected to be high for some reason, the developer is not replenished for a while and ordinary image formation is carried out continuously until the toner concentration inside the developing device returns to the appropriate reference toner concentration. When the toner concentration inside the developing device has returned to the reference toner concentration, the amount of the toner inside the developing device, that is, the amount of the developer, has become scarce, and the volume level of the developer inside the developing device lowers. The fact that the volume level of the developer inside the developing device is lower than a predetermined value indicates that the developer inside the developing device is insufficient. A stirring screw is used to stir the developer inside the developing device, and the stirring screw is usually disposed along the developing roller to convey the developer in the longitudinal direction of the developing roller while stirring the developer. When the developer is conveyed using the stirring screw in this state, the low concentration portion of the developer is also moved as the spiral of the screw is moved, whereby uneven supply to the developing roller corresponding to the movement of the screw occurs. As a result, the influence of the uneven supply of the toner appears on formed images. Hence, in the conventional trickle developing device, the so-called screw irregularity phenomenon reflecting the uneven supply of the toner due to the use of the stirring screw occurs, and there is a problem of being unable to maintain high-quality images.
Accordingly, the technical problem to be solved by the present invention is to provide a developing device and an image forming apparatus capable of carrying out excellent image formation for a long period by making the fluctuations in the toner concentration and the volume level of the developer inside a trickle developing device that uses a two-component developer as small as possible.